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(1) Field of the Invention
This invention relates to a materials-processing system for applying controlled pulses of radiation to materials for making physical changes, and more particularly relates to a highly uniform, high-efficiency optical illumination system which is highly controllable in shape, intensity, and pulse duration to achieve high resolution imaging for materials processing.
(2) Description of Related Art
Several techniques have been developed for converting thin amorphous silicon films into polycrystalline films. See Im and Sposili, Materials Research Society Bulletin, 21, 39(3) (1996). This is desirable because better-performing thin-film transistor (TFT) devices can be fabricated from crystalline silicon than from amorphous silicon, and it is not possible to deposit silicon films directly in a high-quality crystalline state. Among the various crystallization techniques, those based on excimer laser irradiation (or pulsed-laser irradiation more generally) are prevalent because they are compatible with substrates that cannot withstand high-temperature processing; the techniques can be applied when the silicon film to be crystallized is deposited on such a substrate. Glass and other substrates that cannot withstand high-temperature processing (e.g., plastics) are required for use in many TFT applications, for example, in displays.
Sequential lateral solidification (SLS) is a particular type of excimer laser crystallization process that can produce previously unavailable large-grained and grain-boundary-location-controlled microstructures in thin silicon films. The technique is well documented; see Sposili and Im, Applied Physics Letters, 69, 2864 (1996); Sposili and Im, Applied Physics A, A67, 273 (1998); Im, Sposili et al., Applied Physics Letters, 70, 3434 (1997); Sposili, Crowder et al., Mater. Res. Soc. Symp. Proc., 452, 953 (1997); and Im, Crowder et al., Physica Status Solidi A, 166, 603 (1998). Films having these microstructures are superior to the types of random-microstructure polycrystalline silicon produced by other excimer laser crystallization processes (and by non-excimer-laser-based crystallization processes as well) in that the TFTs exhibit a combination of superior electronic performance (e.g., higher carrier mobility) and a high level of device-to-device uniformity. The availability of high-performance TFT devices enables numerous applications, such as integrated active-matrix liquid-crystal displays (IAMLCDs), where the driver circuits and other electronics are integrated directly onto the substrate along with the pixel-controlling transistors, and active-matrix organic light-emitting displays (AMOLEDs), among others.
SLS requires that with each irradiation the silicon film be completely melted in and only in a micron-sized spatially controlled region or regions, and that the film be translated with sub-micron precision in between irradiations such that the lateral crystallization induced by each irradiation overlaps with that produced previously. Various schemes have been proposed for conducting the SLS process, and different approaches exist for effecting the requisite spatial tailoring of the beam. Projection irradiation is a very flexible method, wherein a patterned mask is imaged onto the film in order to define the location and extent of the molten zones. Generally, any projection SLS system would contain the following elements: a pulsed-laser source, typically an excimer laser; an illumination system, including a homogenizer, to provide uniform illumination of the mask; an imaging system to image the mask pattern onto the film, typically at 5:1 or greater reduction, although 1:1 imaging is a possibility; and a high-precision sub-micron sample-translation system.
Such SLS projection systems are similar in many respects to photolithography and ablation systems based on excimer lasers; the basic components listed above are common to all. While the various components and subsystems have the same general purpose for each type of projection system, the requirements of the processes differ and therefore so does the configuration of the subsystems. For example, a system intended for photolithography might require imaging resolution on the order of 1 xcexcm, whereas a resolution of 3-5 xcexcm is usually adequate for SLS. Conversely, SLS has more-stringent requirements than photolithography and ablation in other respects.
An excimer laser projection system designed for SLS requires a high fluence-sufficient to completely melt a silicon film in the exposed region or regions. Spatially homogenous illumination is also essential so that all of the irradiated areas are irradiated at a fluence greater than the complete-melting threshold of the film. If the intensity is too low in particular regions, the film will not melt completely, leading to failure of the process; if the intensity is too high, film damage can occur.
For certain high-throughput configurations of SLS, it would be beneficial to be able to configure the illumination into a high-aspect-ratio form (e.g., a long, narrow rectangle) while maintaining a high degree of spatial uniformity. Techniques describing an internally reflective homogenizer, using a polygonal cross-section, fully internally mirrored chamber to convert collimated light into self-luminous light at an output aperture have been reported in Jain, U.S. Pat. No. 5,059,013 and Farmiga, U.S. Pat. No. 5,828,505. These homogenizer designs offer the additional benefit of preserving numerical aperture (NA) of the illumination, which allows for high optical efficiency. Further, the ability to vary the configuration of the illumination between the high-aspect-ratio shape and some other polygonal shape would also be extremely useful, as it would enable the equipment to be reconfigured between the high-throughput and other variants of the SLS process. For example, a homogenizer with a rectangular cross-section could be constructed wherein one of the sides would be adjustable, enabling one dimension of the rectangle (and thus the aspect ratio) to be varied. An system with such a homogenizer could conveniently be reconfigured to perform different variants of the process as desired. Existing illumination and homogenization schemes that have been applied to SLS cannot provide the requisite level of spatial uniformity in a high-aspect-ratio shape, and are generally limited to providing square or near-square illumination.
The beam area over which a sufficiently high fluence can be maintained is defined primarily by the energy output of the excimer laser source, but for a given laser, fluence can be optimized by a highly efficient optical system, especially one incorporating an energy-recycling scheme. Large-field homogeneous illumination is a prerequisite for a large-working-area beam, which is necessary in order that the process have a high throughput.
A high-efficiency energy-recycling exposure system has been reported. See Hoffman and Jain, U.S. Pat. No. 5,473,408. This scheme offers the additional feature, by the very nature of the energy-recycling scheme with its multiple reflections of portions of the pulse through the optical system, of extending the effective duration of the excimer laser pulse reaching the substrate. Pulse extension beyond the approximately 30-ns FWHM (full-width half-maximum) typically provided by most commercially available excimer lasers offers two benefits to the SLS process: (1) the longer pulse provides some amount of substrate heating, which delays the onset of nucleation during solidification, allowing for a longer lateral growth distance, and therefore increases the throughput of the process; and (2) the substrate heating reduces the thermal gradients during solidification, which in turn reduces the number of intragrain defects, further improving the quality of the crystallized films.
The energy-recycling scheme requires that the illumination and imaging systems be near-telecentric. Telecentricity provides the additional benefits of more-uniform illumination and more-uniform imaging over the entire respective fields compared with non-telecentric optical systems.
Furthermore, given the ability to produce high-aspect-ratio illumination, or more generally, arbitrarily shaped illumination, one could consider imaging the illumination directly without having to use a mask. Such direct imaging of the illumination would significantly increase the efficiency of the imaging system because all of the illumination would be used to irradiate the film; none of the beam energy would be blocked by the mask.
Accordingly, there exists a need in the field for a projection-irradiation-based SLS excimer laser crystallization system incorporating all of the elements and features listed above, with special emphasis on the use of the novel configurable illumination system.
The invention is a system for materials processing with projection irradiation using a pulsed laser source. The material, commonly referred to as the substrate, is made available at a work station for processing by a patterned laser beam. The system typically includes mechanisms for positioning and translating large-area substrates. The positioning may be static, but generally includes a schedule of scanning and stepping that is coordinated with the irradiation in a manner consistent with the requirements of the particular process being conducted. During the scanning motion, the system at the same time provides controlled laser pulses, modifying the laser radiation in an illumination subsystem, and projecting the patterned, modified laser radiation along an optical path to a carefully controlled area on the surface of the substrate.
The output of the laser pulse is controlled in energy and repetition rate, and supplied to the illumination system, which attenuates the beam energy as necessary, reshapes and homogenizes the beam, and provides uniform self-luminous illumination in the desired shape. The homogenizer is an optical tunnel with fully mirrored walls and a polygonal cross-section. Many configurations are available, depending on the desired shape of the illumination. The homogenizer can be constructed so as to be reconfigurable; for example, a rectangular cross-section with one moveable side so that the aspect ratio of the illumination can be adjusted. The internally mirrored surfaces of the homogenizer provide multiple reflections; the mixing of the rays and the resulting randomization of the beam within the homogenizer converts the incident beam into spatially homogenized, self-luminous illumination at the output aperture. The energy-recycling subsystem recovers energy reflected off of the mask that otherwise would have been lost and reintroduces it into the illumination. The re-reflected portion of the beam passes along the optical path two additional times, and depending upon the length of the optical path, is delayed approximately 3.3 ns per meter of beam travel. When all of the secondary, tertiary, and higher-order re-reflected pulses are superimposed on the original pulse, the resulting 5-10-ns delay per re-reflection extends the laser pulse duration by 15-30%. Otherwise-wasted laser power is conserved, and there is an opportunity to control pulse duration quite effectively. The entire optical path up to the mask, with the illumination subsystem including the energy-efficient recycling homogenizer, forms an optical pulse stretcher.
In the case where the output of the homogenizer is imaged directly without employing a patterned mask to define the beam features, a pulse-extender plate (PEP), consisting of an essentially transparent but partially reflective material such as fused silica, can be used to reflect some of the beam energy back into the energy recycler in order to extend the pulse duration. In either case (using a mask or a PEP), the exact amount of temporal extension can be controlled by adjusting the optical length of the pulse stretcher and by engineering the reflectivity of the mask or PEP, for example through the use of partially reflective dielectric coatings.
The imaging system projects the image of the mask, or in the case of the maskless system, of the illumination itself onto the substrate. A significant portion of the patterned beam energy is absorbed by the substrate and effects a physical change, usually through a thermal mechanism as a result of localized intense heating. However, other modes of interaction between the beam and the substrate are possible, depending on the specific substrate material and the wavelength of the laser radiation.
The localized intense heat is useful in many materials-processing operations, including those involving localized melting and resolidification, such as excimer laser annealing (ELA) in general and SLS in particular, and dopant activation, as well as those involving localized ablation and removal of material.
The object of the invention is to provide a high-fluence, shaped, spatially homogenized, duration-controlled, and carefully imaged radiation beam at the chosen area or areas of the substrate, which can be translated in an extremely precise and controllable manner in coordination with the irradiation, in order to accomplish the desired physical change.
A feature of the invention is the use of a novel optical system, including an illumination subsystem to produce a self-luminous beam of selected cross-section, spatially uniform intensity, and selected numerical aperture, as well as a high-efficiency energy-recycling exposure system, which controls the duration of radiation pulses and conserves energy.
An advantage of the invention is that the desired physical changes to the substrate can be controlled and the processing can be optimized (e.g., for high throughput) by full control of the applied irradiation; by controlling fluence, beam shape, homogenization, pulse duration, image pattern, and translation schedule of the substrate with respect to the beam.
The invention is a system for conducting the sequential lateral solidification (SLS) process on substrates, including large-area substrates, containing a thin surface layer of material to be crystallized, typically but not limited to amorphous silicon (a-Si).
The purpose is to create desirable microstructures in the film, such as large-grained, grain-boundary-location-controlled polycrystals and large single-crystal islands at selected locations on the substrate.
It is the object of the invention to provide a novel optical system including: (1) an illumination system to produce a self-luminous beam of selected cross-section, spatially uniform intensity, and selected numerical aperture; (2) a high-efficiency energy-recycling exposure system to control the pulse duration and to make efficient use of the beam energy; (3) telecentricity of both illumination and imaging systems for improved uniformity of illumination and imaging over large fields; and (4) high-resolution imaging of features onto the film.
Another object of the invention is to provide a practical system for conducting SLS on large substrates using the novel optical system in conjunction with a high-precision, large-travel translation system.
Another object of the invention is to provide a practical system for pulsed-laser electronic activation of dopants either concurrently with SLS processing for pre-doped films, or as a separate processing step for films doped after being crystallized.
A feature of the invention is the use of a novel illumination system to produce a self-luminous light beam of selected cross-section, spatially uniform intensity, and selected numerical aperture.
Another feature of the invention is the use of a high-efficiency energy-recycling exposure system, which controls the duration of radiation pulses and conserves energy.
An advantage of the invention is the ability to configure the homogenous illumination into various shapes, including a high-aspect-ratio polygon, which is well-suited for certain high-throughput variants of the SLS process.
Another advantage of the invention is the ability to reconfigure the shape of the illumination so that different variants of the SLS process can be conducted on different areas of the substrate, so as to optimize the microstructure and throughput as appropriate for the desired application.
Another advantage of the invention is the ability to adjust the pulse duration so as to (1) increase the lateral growth distance and in turn the throughput of the process, as well as (2) reduce the number of intragrain defects forming during solidification and in so doing improve the quality of the crystallized films.
Another advantage of the invention is that the same tool may be used for both SLS and post-doping annealing (dopant activation).
Other objects, features and advantages of the invention will be apparent from the following written description, claims, abstract, and the annexed drawings.